Deep venous thrombosis (DVT) of the lower extremity is one of the major causes of morbidity and mortality in hospital based patients. When thrombi form and then break off, they can travel to the lungs, a condition known as pulmonary embolism, which represents leading cause of preventable death in the United States. Identifying people who are at risk for DVT and pulmonary embolism allows clinicians to determine who should get life saving prophylactic therapy to protect against PE. Studies have found several factors that are associated with DVT including recent surgery, malignancy, smoking and oral contraceptive usage. Since the time of Virchow in the 1800s, investigators have noted that DVT tends to occur more often in the left leg versus the right leg. Pathologists doing cadaver dissections noted that the artery that supplies blood to the right leg can often crossover and compress the vein that is returning blood from the left leg to the heart. May and Thurner, two pathologists, first noted that webs of fibrous tissue formed at the location where the artery compressed the vein. They suggested that compression and chronic pulsation of the artery compressing the vein leads to injury of the vein, causing the formation of the webs, predisposing people to thrombosis. For their discovery, the combination of vein compression leading to thrombosis has been called May Thurner syndrome. Several studies have used CT, MRI and ultrasound imaging to detect high levels of compression in patients with left DVT. However one recent paper by Kibbe (JVS 2004) suggested that some degree of vein compression occurs in nearly 2/3 of the population, raising the question if vein compression is actually a risk factor for left DVT or if it is merely a normal variant. We hope to determine if iliac vein compression is indeed a risk factor for DVT and how it might interact with other known risk factors to cause thrombosis. We plan to utilize epidemiologic methods for studying risk factors, employing a case control design of hospital-based patients who have DVTs in their left leg compared with controls who have DVTs in their right leg. Additionally we aim to determine the interaction of iliac vein compression with other known risk factors for DVT such as cancer and genetic mutations. PUBLIC HEALTH RELEVANCE: DVT is one of the leading risk factors for death in hospitalized patients. Recent studies suggest that imaging may be useful in classifying patients who may be at risk for thrombosis. With this grant, we hope to use new imaging techniques to determine who is at risk for DVT so that we can prevent people from developing DVT.